A round-trip signal propagation time in a small “home” network is likely to be considerably smaller than a corresponding time in an “external” network, such as an internet-like network. In the “external” network propagation delays through switching elements, store-and-forward nodes, and so forth are likely to be much longer than propagation delays in the “home” network. A good estimate of an in-home round-trip propagation delay is less than approximately 10 milliseconds, whereas an external network will have a considerably longer round-trip propagation delay. Such a difference in propagation delay between an “external” network and a “home” is not only due to differences in physical distances within the networks, but also, and mainly, due to additional “hops” between network routers and other network elements.
Propagation delay by itself is not an acceptable measure of proximity between network elements because of the following reasons:                It would be very easy for an eavesdropper or a hacker to create a propagation delay measurement deception,        It is difficult to separate a “network” propagation delay (due to distance and intermediate agents) from a processing delay which is due to processing time in hardware and/or software of each network element.        
Published PCT application WO 01/93434 of Xtemespectrum, Inc. describes a method, a device and a computer readable medium for enabling and blocking communications with a remote device based on a distance of the remote device. The method on which the device and computer readable medium are based includes transmitting a message from a local device to remote device via ultra wide band (UWB) wireless medium and receiving a response from the remote device via the UWB wireless medium. The transmitting and receiving steps are preferably performed in accordance with a Media Access Control (MAC) protocol. A distance between the local device and the remote device is then determined based on a time between the transmitting the message and the receiving of the response and a function, such as communicating with the remote device, is preformed in the local device based on the distance determined. The communication between the local device and the remote device may be enabled or disabled depending on the distance that the remote device is from the local device.
Published PCT application WO 02/35036 of Volvo Teknisk Utveckling AB describes a method for controlling authorization for access to an object, in which a signal communication via electromagnetic waves is established between the object and a wireless portable unit when a tripping device on the object is actuated. The signal communication comprises at least one first signal that is sent from the object to the portable unit, and at least one second signal that is sent from the portable unit to the object in response to the first signal. The second signal comprises sufficient information for verifying that the portable unit has an approved identity. The verification information is checked, a distance is measured between the object and the portable unit and the authorization is confirmed if both the checked verification information is approved is approved and the measured distance is less than a predetermined value. For the distance measurement, a time is measured for the transmission of at least one of the first signals and at least one of the second signals with verification information.
Published US Patent Application 2002/0087666 of Huffman et al. describes a Method for locating logical network addresses on electronically switched dynamic communications networks, such as the Internet, using the time latency of communications to and from the logical network address to determine its location. Minimum round-trip communications latency is measured between numerous stations on the network and known network addressed equipment to form a network latency topology map. Minimum round-trip communications latency is also measured between the stations and the logical network address to be located. The resulting set of minimum round-trip communications latencies is then correlated with the network latency topology map to determine the location of the network address to be located.
Published US Patent Application 2003/0046022 of Silverman describes a method for determining the physical location of a target device. Using communications network trace route and pinging commands, the distances of three test devices of known locations to the target device are determined; and responsive to those distances, the location of the target device is determined by triangulation. Based upon location, the target device may be blocked from a communications network or connected to a particular server.
Published PCT application WO 2004/014037 of Koninklijke Philips Electronics N.V. describes a method for a first communication device performing authenticated distance measurement between the first communication device and a second communication device. The first communication device and second communication share a common secret, the common secret is used for performing the distance measurement between the first and second communication devices.
Published PCT application WO 03/079638 of Koninklijke Philips Electronics N.V. describes a method for determination of proximity between nodes based on the communication time between the nodes. A source node communicates a query to a target node. The target node is configured to automatically send a response to the sender of the query. The communication time is determined based on the time duration between the transmission of the query and receipt of the response at the source node. The communication time is compared to a threshold value b determine whether the target node is local or remote relative to the source node.
Published PCT application 2004/030311 of Koninklijke Philips Electronics N.V. describes a method for determining the proximity of a target node to a source node from a response time required to communicate messages within a node-verification protocol. The node-verification protocol includes a query response sequence, wherein the source node communicates to a target node, and the target node communicates a corresponding response to the source node. The target node is configures to communicate two responses to the query: a first response that is transmitted immediately upon receipt of the query, and a second response based on the contents of the query. The communication time is determined based upon the time duration between the transmission of the query and receipt of the first response at the source node and the second response is compared for correspondence to the query, to verify the authenticity of the target node.
Published PCT application 2004/030312 of Koninklijke Philips Electronics N.V. describes a method including timing parameters within a node-verification protocol to determine the proximity of a target node to a source node. The node-verification protocol includes a query response sequence between the source node and the target node. The source node establishes a lower bound on the distance between the source node and the target node based on a measure of the time required to effect this query-response sequence including the time required to communicate the query and response, as well as the time required to process the query and generate the response to the source node. The target node includes a measure of the time required to process the query and generate the response to the source node. The source node subtracts this time from the total query-response time to determine the time consumed for the communication.
The Secure Video Processor (SVP) Alliance is a group which offers a standard secure method for digital content protection providing new opportunities for content delivery while ensuring protection for content owners, and low cost and ease of use for consumers. More information about the SVP Alliance, and SVPs is available on the World Wide Web at www.svpalliance.org. The information disclosed in the document found on the SVP Alliance website at www.svpalliance.org/docs/FAQ.pdfis hereby incorporated herein by reference.
The disclosures of all references mentioned above and throughout the present specification, as well as the disclosures of all references mentioned in those references, are hereby incorporated herein by reference.